@Eklispe Sorry for the holdup, some communication hiccups between me and Sat made me unwilling to answer any sooner.
So to your first question, the characters
themselves decide who teams up with who. If your cast gets along well with my cast, for example, there's a real chance they'd decide to team up. The same applies to anyone looking to ally with Satoshi's crew. Conversely, I doubt the Synchro or Water dorm would team up, considering they showed hostility to each other so far.
To answer the second, I don't recommend trying it. You can be a sneaky snitch and do it anyway, but if either dorm found out you're attempting to double-time them, I doubt they'd want to be your ally for much longer.